Removal of anionic azo dyes from aqueous solution by functional ionic liquid cross-linked polymer

Removal of anionic azo dyes from aqueous solution by functional ionic liquid cross-linked polymer

Journal of Hazardous Materials 261 (2013) 83–90 Contents lists available at SciVerse ScienceDirect Journal of Hazardous Materials journal homepage: ...

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Journal of Hazardous Materials 261 (2013) 83–90

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Removal of anionic azo dyes from aqueous solution by functional ionic liquid cross-linked polymer Hejun Gao a,b , Taotao Kan c , Siyuan Zhao d , Yixia Qian d , Xiyuan Cheng d , Wenli Wu d , Xiaodong Wang e , Liqiang Zheng a,∗ a

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong 637000, China c CNOOC Energy Technology and Services-oilfield Technology Services Co., Tanggu, Tianjin 300452, China d School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, China e Shandong Provincial Analysis and Test Center, Jinan 250100, China b

h i g h l i g h t s • • • •

Equilibrium, kinetic and thermodynamic of adsorption of dyes onto PDVB-IL was investigated. PDVB-IL has a high adsorption capacity to treat dyes solution. Higher adsorption capacity is due to the functional groups of PDVB-IL. Molecular structure of dyes influences the adsorption capacity.

a r t i c l e

i n f o

Article history: Received 23 March 2013 Received in revised form 1 July 2013 Accepted 2 July 2013 Available online xxx Keywords: Azo dye Adsorption Ionic liquid Polymers

a b s t r a c t A novel functional ionic liquid based cross-linked polymer (PDVB-IL) was synthesized from 1-aminoethyl3-vinylimidazolium chloride and divinylbenzene for use as an adsorbent. The physicochemical properties of PDVB-IL were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy and thermogravimetric analysis. The adsorptive capacity was investigated using anionic azo dyes of orange II, sunset yellow FCF, and amaranth as adsorbates. The maximum adsorption capacity could reach 925.09, 734.62, and 547.17 mg/g for orange II, sunset yellow FCF and amaranth at 25 ◦ C, respectively, which are much better than most of the other adsorbents reported earlier. The effect of pH value was investigated in the range of 1–8. The result shows that a low pH value is found to favor the adsorption of those anionic azo dyes. The adsorption kinetics and isotherms are well fitted by a pseudo secondorder model and Langmuir model, respectively. The adsorption process is found to be dominated by physisorption. The introduction of functional ionic liquid moieties into cross-linked poly(divinylbenzene) polymer constitutes a new and efficient kind of adsorbent. © 2013 Elsevier B.V. All rights reserved.

1. Introduction As the largest and most versatile class of organic dyestuffs, azo dyes are widely used in textile, paper, and leather industry [1]. With the development of the industries above, a huge amount of wastewater containing azo dyes is discharged to the environment above the level that the nature can eliminate. The azo dyes and their breakdown products can cause toxic effects in the aquatic environment and are mutagenic and carcinogenic to humans [2–4]. The treatment of dye wastewater becomes more important than ever for the environment. Many technologies were applied to

∗ Corresponding author. Tel.: +86 531 88366062; fax: +86 531 88564750. E-mail address: [email protected] (L. Zheng). 0304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2013.07.001

treat dye wastewater, such as biological treatment [5], coagulation/flocculation [6], chemical oxidation [7], membrane filtration [8], ion-exchange [9], photocatalytic degradation [10], and adsorption [11]. Among those technologies, the most common one is the adsorption technology due to its effectiveness, efficiency, economy and no secondary pollution. Ionic liquids (ILs) are receiving much attention owing to their unique properties, such as high thermal stability and high ionic conductivity [12]. Currently, many functionalized ILs have been used to protect water resource. Fuerhacker et al. [13] reported that the degree of removal of heavy metals (Cu, Ni and Zn) can reach 90% using quaternary ammonium and phosphonium ILs. Gharehbaghi and Shemirani [14] prepared an ionic liquid (1-hexyl3-methylimmidazolium bis(trifluormethylsulfonyl) imid). When it was injected into the Congo Red wastewater, most of the

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dye molecules were extracted into fine IL droplets and removed from aqueous phase. Poursaberi and Hassanisadi [15] synthesized [email protected] O4 nanoparticles from IL-COOH and Fe3 O4 and found it to be an effective adsorbent of Reactive Black 5 (maximum adsorption capacity 161.29 mg/g). The preparation of an effective adsorbent for removing anionic azo dyes from aqueous solution is expected to meet the demand of environmental protection. It is well known that the aminefunctional groups on the surface of an adsorbent can greatly improve adsorption capacity [16,17]. The adsorbent with multiple benzene rings can bring – stacking interaction between azo dye and the adsorbent, which further improves the adsorption capacity [18]. The ILs with amine groups on cross-linked polymer containing multiple benzene rings may obtain a new kind of efficient adsorbent. In this work, a cross-linked polymer is obtained by copolymerizing divinybenzene and imidazolium ILs with an amine group. The structure of the cross-linked polymer was characterized and the physicochemical properties were investigated in detail. The adsorption capacity of functional ionic liquid cross-linked polymer was investigated in anionic azo dyes solutions.

Table 1 Characteristics and structures of anionic azo dyes. Generic name

M.W. max (nm)Molecular structure

Orange II

350.32 484

Sunset yellow FCF452.38 482

2. Experimental 2.1. Materials Orange II (85%), sunset yellow FCF (95%), and amaranth (98%) were obtained from Aladdin Chemical Reagent Co., Ltd. Those dye molecular structures are shown in Table 1. 1Vinylimidazole (99%), 2,2 -azobis(2-methylpropionitrile) (99%), dimethylformamide (99%) and 2-chloroethylamine hydrochloride (99%) were purchased from Beijing Chemical Reagent Co. Divinylbenzene (DVB) (80%) was provided by Aldrich. Amaranth

604.47 521

2.2. Methods 2.2.1. Preparation of cross-linked polymer [19] 1-Vinylimidazole (0.1 mol) and 2-chloroethylamine hydrochloride (0.1 mol) were added into 50 ml acetonitrile. The mixture was stirred and refluxed under a nitrogen atmosphere for 48 h. The resulting solid was washed several times with anhydrous ethanol. The solid with equimolar NaOH were dissolved in water and stirred at room temperature for 24 h. After evaporating, the crude product was extracted by methanol. Then, the product was dried for 48 h under vacuum and the functional ionic liquid was obtained. DVB (0.02 mol), 1-aminoethyl-3-vinylimidazolium chloride (0.005 mol) and an appropriate amount of 2,2 -azobis(2methylpropionitrile) were dissolved in 50 ml dimethylformamide under nitrogen. The mixture was stirred at 80 ◦ C for 24 h, and a yellow solid was collected by filtration and washed with acetone. The adsorbent of poly(divinylbenzene-co-1-aminoethyl3-vinylimidazolium chloride) was obtained. The synthetic process of PDVB-IL is shown in Fig. 1. 2.2.2. Characterization of the adsorbent The Fourier transform infrared (FT-IR) spectra of PDVB-IL, PDVBIL-Orange II (after adsorption of orange II on PDVB-IL) and orange II were obtained on VERTEX-70 FT-IR spectrometer using KBr pellet in the range of 4000–400 cm−1 . The particle size and morphology of PDVB-IL were characterized using a scanning electron microscope (SEM, JSM-7600F, EOL, Ltd., Japan). The N2 adsorption–desorption isotherm for sample was measured at 77 k with a Quadrasorb Station 2 Analyzer (Quantachrome, USA). Thermogravimetric analysis (TGA) was carried out using a Rheometric Scientific TGA1500 (Piscataway, NJ) to investigate the thermal properties of samples. Studies were conducted under inert atmosphere of nitrogen using

8–10 mg samples at a heating rate of 10 ◦ C/min in the range of ambient temperature to 800 ◦ C. 2.2.3. Batch adsorption procedure Batch adsorption was carried out in order to evaluate the adsorption capacity. The three dye solutions were prepared by dilution of the stock standard solution (dye solution 200 mg/L). All batch adsorption experiments were carried out as follows: 20 mL dye solutions and 2 mg PDVB-IL were poured into 100 mL conical beaker. The conical beaker was shaken by the oscillator (SHZ-82, Changzhou Shaipu Experimental Instrument Factory, China) at a speed of 150 rpm. After centrifugation, the solution absorbance was measured using UV–vis–NIR spectroscopy (Hitachi U-4100, Japan) with the max at 484, 482 and 520 nm for orange II, sunset yellow FCF and amaranth, respectively. The amount of adsorbed dye on PDVB-IL (q, mg/g) was calculated according to the following equation: q=

C0 − Cf m

V

(1)

where C0 is the initial concentration of dyes in solution (mg/L), Cf is the dyes concentration at equilibrium (mg/L), m is the mass of adsorbent (g), and V is the volume of solution (L).

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Fig. 1. Synthetic route of PDVB-IL.

where Ce is the equilibrium liquid concentration (mg/L), qe is the amount adsorbed on solid at equilibrium (mg/g), qm is the maximum theoretical adsorption capacity (mg/g), and KL is the adsorption equilibrium constant (L/mg). The constants qm and KL can be calculated from the intercepts and the slopes of a linear plots of Ce /qe versus Ce . The Freundlich isotherm is the most important multilayer adsorption isotherm for rough surfaces. The linear form of the Freundlich isotherm model can be represented as log qe =

Fig. 2. FT-IR spectra of PDVB-IL, PDVB-IL-Orange II (i.e. after adsorption of Cr(VI) onto PDVB-IL) and orange II.

2.2.4. Models for adsorption kinetics, isotherm and thermodynamics To investigate the amount of anionic azo dyes adsorbed on the surface of the PDVB-IL at any time and the amount adsorbed at equilibrium, the pseudo-first-order and pseudo-second-order models were adopted to elucidate the adsorption kinetic process, which could be expressed as Eqs. (2) and (3), respectively, log(qe − qt ) = log qe − k1 t

(2)

1 t t = + qt qe k2 q2e

(3)

where k1 (/h) and k2 (mg/g h) are the rate constant of the pseudo first-order and the pseudo-second-order kinetics, respectively. qt (mg/g) and qe (mg/g) represent the adsorption capacity at any time t (h) and at equilibrium, respectively. t (h) is the contact time. The dyes adsorption isotherm data were correlated with the theoretical models of Langmuir and Freundlich. The Langmuir isotherm model is obtained by Ce 1 Ce = + qe qm KL qm

(4)

1 log Ce + log KF n

(5)

where KF and 1/n are characteristic constants representing the adsorption capacity and adsorption intensity of the system, respectively. The values of KF and 1/n are obtained from linear plots of log qe versus log Ce . For the Langmuir-type adsorption process, the essential characteristics can be explained by dimensionless constant separation factor (RL ), which is considered as a more reliable indicator of the adsorption capacity. The value of RL can be obtained by the following equation: RL =

1 1 + KL C0

(6)

The effect of temperature on the adsorption of anionic azo dyes was studied in order to obtain the relevant thermodynamic parameters. The standard free energy change (G◦ ) for adsorption can be calculated from the following equation: G◦ = −2.303RT log Ke

(7)

where R is the universal gas constant (8.314 J/mol K) and T is the absolute temperature (K). The thermodynamic parameters of standard enthalpy change (H◦ ) and entropy change (S◦ ) for the process can be determined from the following equations: log Ke = Ke =

qe Ce

S ◦ H ◦ − 2.303R 2.303RT

(8)

(9)

The values of H◦ and S◦ can be calculated from the slope and the intercept, respectively, of a linear plot of logKe against 1/T.

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Fig. 3. The interaction between dye molecules and PDVB-IL.

The activation energy (Ea ) and sticking probability (S*) for the adsorption of anionic azo dyes onto the surface of the PDVB-IL are calculated by S∗ = (1 − )e−(Ea /RT )

(10)

The sticking probability (S*) is a function of the adsorbate/adsorbent system under investigation but must lie in the range 0 < S* < 1 and is dependent on the temperature of the system. The surface coverage () can be calculated from the following equation:  =1−

Ce C0

(11)

3. Results and discussion 3.1. Characterization of PDVB-IL The FTIR spectra of PDVB-IL, PDVB-IL-Orange II (i.e. after adsorption of Cr(VI) onto PDVB-IL), and orange II are shown in Fig. 2. In the spectrum of PDVB-IL, the peak at 3440 cm−1 is attributed to the stretching mode of the N H groups. The bands at 2926 and

2867 cm−1 are assigned to the C H stretching vibrations [20]. The bands at 1630 and 1603 cm−1 are due to C C stretching vibrations of imidazole ring and N H inplane bending vibrations, respectively [21]. A strong band at 1448 cm−1 is assigned to C C stretching of the benzenoid rings [22]. The peak at 1162 cm−1 is C N stretching vibrations. In addition, the spectrum shows the presence of aromatic (900–600 cm−1 ) groups [23]. The above results indicate that the cross-linked polymer containing the functional groups of amine and benzene ring was successfully prepared. The peak at 3441 cm−1 (PDVB-IL) is shifted to 3454 cm−1 in the spectrum of PDVB-ILOrange II, which can be due to the hydrogen bonding interactions between PDVB-IL and orange II. Comparison of the FT-IR spectra of orange II and PDVB-IL-Orange II could reveal that absorption peaks of PDVB-IL-Orange II in the bands of 4000–3000, 1700–1400, and 900–600 cm−1 shifted to longer wavelengths, indicating that – interactions exist. The bands at 1035–1204 cm−1 of –SO3 − (Orange II) and 1162 cm−1 of C N (PDVB-IL) are reduced after adsorption of orange II onto PDVB-IL, which reflects the effect of electrostatic interaction on the binding of orange II. The interaction between orange II and PDVB-IL is shown in Fig. 3. SEM images of the surface of PDVB-IL are depicted in Fig. 4. The surface of PDVB-IL is rough. The specific surface area BET value is 655.051 m2 /g (Fig. 5A) and the pore diameter of PDVB-IL is 2.583 nm (Fig. 5B). The large, rough and porous surface provides a good adsorption site for dyes to be trapped and adsorbed. The thermal stability of prepared PDVB-IL was characterized by thermogravimetric analysis. TGA plots of PDVB-IL is shown in Fig. 6. The PDVB-IL shows three weight losses, which appears at ∼300–410 ◦ C, ∼410–500 ◦ C and above 500 ◦ C. The weight loss of PDVB-IL at ∼300–410 ◦ C is due to the part of ILs on the polymer [24]. The main weight loss occurring between 430 and 500 ◦ C is attributed to degradation of PDVB [19]. The amount of ILs in the PDVB-IL is about 6.9 wt%. The weight loss of PDVB-IL is negligible below 300 ◦ C, indicating that the PDVB-IL can be applied to treat high temperature wastewater. 3.2. Effect of pH on adsorption The pH of initial solution has significant effect on the adsorption process, since it determines the surface charge of the adsorbent and the degree of ionization and speciation of the adsorbates [16,25]. The functional group of amine (–NH2 ) on PDVB-IL can be protonated to ammonium cation with decreasing pH value of solution [26]. The adsorption capacities of dyes on PDVB-IL are plotted as a function of pH value in Fig. 7. The results show that the adsorption capacity of anionic azo dyes gradually increases with decreasing pH from 8 to 1. This may be due to the increase of strong electrostatic attractions induced by ammonium cationic group with decreasing pH, which leads to the increase of adsorption capacity [27,28]. The number of negative charge per dye molecule (orange II, sunset

Fig. 4. SEM images of PDVB-IL.

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Fig. 5. (A) N2 sorption isotherms and (B) pore size distributions for PDVB-IL.

Fig. 6. TGA of PDVB-IL under nitrogen.

yellow FCF, and amaranth) is different. One SO3 − (orange II) needs one cationic site, while one molecule of sunset yellow FCF and amaranth need two and three cationic sites, respectively. Therefore, the capacity of different dyes adsorption is different at the same pH value. The following experiments were carried out at natural pH (5.6).

Fig. 7. Influence of pH on the adsorption capacity of PDVB-IL toward anionic azo dyes. (C0 = 120 mg/L, T = 25 ◦ C, contact time = 6 h).

Fig. 8. Adsorbed amount of anionic azo dyes by PDVB-IL as a function of contact time. (C0 = 100 mg/L, T = 25 ◦ C).

3.3. Adsorption kinetics The adsorption kinetics of orange II, sunset yellow FCF and amaranth on PDVB-IL were studied to investigate the adsorption rate at which contaminates were removed from aqueous solutions. Fig. 8 shows the effect of contact time on the amount of adsorbed anionic azo dyes at 25 ◦ C. Fig. 8 showed that the three anionic azo dyes were adsorbed rapidly at the initial 1 h. After 5 h, the adsorption capacity did not change with the contact time, indicating the adsorption equilibrium was approached. The adsorption capacity of anionic azo dyes on PDVB-IL at equilibrium are 868.41, 760.52, and 490.51 mg/g for orange II, sunset yellow FCF and amaranth at 25 ◦ C, respectively. The pseudo-first-order and pseudo-second-order kinetic parameters are given in Table 2. The coefficient of determination (R2 ) values of pseudo first-order kinetic curve can reach up to 0.97. However, the experimental qe values do not agree with that calculated ones (Table 2), indicating that the adsorption of anionic azo dyes onto PDVB-IL adsorbent does not conform to the pseudofirst-order model. The R2 values of pseudo second-order kinetic curve are greater than 0.99. Moreover, the values of calculated qe are closer to the experimental qe values for pseudo-second order kinetics, indicating that the adsorption kinetics is well fitted by a pseudo second-order model [29]. Ranking of these anionic azo dyes in terms of k2 order: Orange II > Sunset yellow FCF > Amaranth

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Table 2 Pseudo-first-order and pseudo-second-order kinetic parameters for adsorption rate of anionic azo dyes on PDVB-IL. Component Orange II Sunset yellow FCF Amaranth

Pseudo-first-order qe (mg/g)

K1 × 10−1 (/h)

R2

Pseudo-second-order qe (mg/g) K2 × 10−3 (mg/g h)

R2

251.49 ± 18.14 321.73 ± 20.60 433.58 ± 43.56

2.00 ± 0.17 2.05 ± 0.15 2.85 ± 0.24

0.9649 0.9736 0.9663

892.91 ± 7.12 800.10 ± 8.76 558.79 ± 8.47

0.9991 0.9984 0.9969

4.83 ± 0.75 3.21 ± 0.41 1.95 ± 0.15

Experimental qe 868.41 ± 23.04 760.25 ± 41.32 490.51 ± 28.32

Table 3 Langmuir and Freundlich constants for the adsorption of orange II, sunset yellow FCF (SY) and amaranth on PDVB-IL. Component

T (◦ C)

Langmuir constants qm (mg/g) KL (L/mg)

R2

Freundlich constants KF (mg/g)

RL

R2

n

Orange II

10 25 40

840.68 ± 17.23 943.45 ± 7.10 970.91 ± 6.41

0.59 ± 0.23 1.29 ± 0.31 1.69 ± 0.44

0.9974 0.9997 0.9997

0.011–0.043 0.005–0.017 0.004–0.013

526.86 ± 23.56 614.89 ± 8.81 652.38 ± 10.04

8.88 ± 1.28 7.78 ± 0.39 8.06 ± 0.48

0.9149 0.9889 0.9846

SY

10 25 40

724.65 ± 3.21 746.35 ± 7.82 787.45 ± 5.91

1.25 ± 0.28 1.72 ± 1.04 2.89 ± 1.86

0.9998 0.9995 0.9997

0.005–0.017 0.004–0.022 0.003–0.013

547.09 ± 6.33 631.57 ± 10.77 688.19 ± 10.31

14.14 ± 0.79 29.42 ± 4.90 34.27 ± 4.86

0.9861 0.8888 0.9168

Amaranth

10 25 40

523.61 ± 5.16 610.29 ± 17.99 690.50 ± 24.18

0.23 ± 0.03 0.11 ± 0.02 0.10 ± 0.02

0.9994 0.9948 0.9927

0.027–0.077 0.056–0.143 0.061–0.156

302.32 ± 9.55 249.29 ± 22.94 237.37 ± 22.93

8.57 ± 0.63 5.54 ± 0.77 4.53 ± 0.56

0.9764 0.9196 0.9363

Table 4 Comparison of PDVB-IL adsorption capacity among different adsorbents. Anionic azo dyes

Adsorbents

Adsorption capacities (mg/g)

References

Orange II

Bottom ash Sludge adsorbent Activated carbon PDVB-IL

12.50 350 404 1000

[30] [31] [32] Present work

Sunset yellow FCF

Mangrove barks LDH-MAN 4 CaAl-LDH-NO3 PDVB-IL

12.72 142.86 398.41 769.23

[33] [34] [27] Present work

Amaranth

Alumina reinforced polystyrene Citrullus lanatus rind Fe3 O4 /ZrO2 /chitosan PDVB-IL

16.86 23.0 99.6 666.67

[29] [35] [36] Present work

In the dyes solution, the anionic dye molecules are trapped by PDVB-IL. The electrostatic repulsion could be formed by the negative charges of dye molecules on the surface of adsorbent, which may decrease the speed of adsorption. One amaranth molecule has three negative charges. The k2 value of amaranth should be the smallest one. In other words, the k2 value of orange II is the biggest one, because it is only one negative charge per molecule. 3.4. Adsorption isotherms Adsorption process is usually studied through adsorption isotherm. The adsorption isotherm describes the equilibrium relationship between the adsorbate, adsorbent, and the equilibrium concentration of adsorbate in solution. Two well-known adsorption isotherms, Langmuir and Freundlich models, are applied to describe the adsorption isotherms. These isotherms relate the equilibrium adsorption capacity to the equilibrium concentration in the solution (Fig. 9). The Langmuir and Freundlich constants can be obtained by linear regression analysis. Table 3 presents the results and the coefficient of determination (R2 ). It can be seen that the R2 values of Langmuir isotherm are greater than those for Freundlich isotherm, indicating that Langmuir model is better fitted the experimental data than Freundlich model at investigated temperatures. The results suggest the monolayer coverage of dye on the surface of PDVB-IL. The qm can reach 925.09, 734.62, and 547.17 mg/g for orange II, sunset yellow FCF and amaranth at 25 ◦ C,

respectively. Adsorption capacities of different adsorbents toward the three anionic azo dyes are compared with PDVB-IL in Table 4. It is obvious that the adsorption capacity of PDVB-IL is much higher than most of the other adsorbents reported earlier. The large adsorption capacity could belong to the strong adsorption affinity of PDVB-IL toward the three anionic azo dyes, which is caused by the high specific surface area coupled with unusual surface morphologies, as well as – stacking, hydrogen-bonding and electrostatic interaction between dye and functional group of PDVB-IL [37,38].

Fig. 9. Adsorption isotherms for anionic azo dyes at 25 ◦ C. (contact time = 6 h).

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Table 5 Thermodynamic parameters for adsorption of anionic azo dyes on PDVB-IL. Compound

Temperature (K)

G◦ (kJ/mol)

H◦ (kJ/mol)

S◦ (kJ/mol K)

Ea (kJ/mol)

S*

283 298 313

−7.197 ± 0.054 −8.712 ± 0.050 −9.488 ± 0.051

14.537 ± 3.450

0.077 ± 0.012

10.742 ± 2.422

0.479 ± 0.401

Orange II

283 298 313

−6.314 ± 0.111 −6.835 ± 0.114 −7.595 ± 0.113

5.720 ± 1.119

0.042 ± 0.004

3.556 ± 0.728

0.011 ± 0.003

Sunset yellow FCF

283 298 313

−4.589 ± 0.110 −5.266 ± 0.105 −6.043 ± 0.099

9.106 ± 0.466

0.048 ± 0.002

4.137 ± 0.312

0.010 ± 0.001

Amaranth

The different maximum adsorption capacities are attributed to the molecular structures of anionic azo dye molecules. When the dye molecules were adsorbed onto PDVB-IL, the negative charges of dye molecules induced electrostatic repulsion on the surface of PDVBIL. With increasing negative charge of dye molecules (Table 1), the electrostatic repulsion became strong and many positive charges on the surface of PDVB-IL were consumed. As a result, the adsorption capacity decreased sharply with the increasing of negative charges per molecule. The value of RL can show that the adsorption dye on PDVBIL is unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0) [39]. The result is shown in Table 3. The RL values for all C0 were between 0 and 1, implying favorable adsorption of three anionic azo dyes on PDVB-IL.

3.5. Thermodynamics of adsorption Table 5 presents the resulting G◦ , H◦ , and S◦ values for adsorption of anionic azo dyes on PDVB-IL. A negative change in adsorption standard free energy (G◦ ) indicates that the adsorption behavior is a spontaneous process [40]. With an increase in temperature, the absolute value of G◦ gradually increases, implying that the adsorption process is more spontaneous at higher temperature. The positive values of H◦ confirms the endothermic nature of the adsorption process, and the positive values of S◦ imply that the adsorbed anionic azo dyes present a certain amount of freedom in the solid/solution interface [41]. This phenomenon is due to the physisorption, which takes place through electrostatic interactions. This interaction is attributed to the amine functional groups on the surface of PDVB-IL. In order to further confirm that the physisorption is the predominant mechanism, the Ea and S* are calculated from the slopes and the intercepts of a linear plots of ln (1−) versus 1/T, and are shown in Table 5. The value of Ea is small, indicating that the physisorption is the dominating role in the adsorption process [42].

4. Conclusions The functional ionic liquid cross-linked polymer, PDVB-IL, was successfully prepared by polymerization of 1-aminoethyl3-vinylimidazolium chloride and divinylbenzene. The adsorptive capacity of PDVB-IL was measured in three kinds of anionic azo dyes solutions (orange II, sunset yellow FCF, and amaranth). With the increase of pH, the adsorption capacity gradually decreases. The maximum values of orange II, sunset yellow FCF, and amaranth are 925.09, 734.62, and 547.17 mg/g at 25 ◦ C, respectively. The adsorption thermodynamics show that physisorption plays the dominating role in the adsorption process. It is hoped that this work may provide a novel kinds of adsorbent with important practical applications.

Acknowledgments The authors are grateful to the National Basic Research Program (2009CB930101), the National Natural Science Foundation of China (No. 91127017), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120131130003) and the Shandong Provincial Natural Science Foundation, China (ZR2012BZ001). References [1] R.J. Chudgar, J. Oakes, Kirk–Othme Encyclopedia of Chemical Technology – Section on Azo Dyes, fifth ed., John Wiley & Sons Inc., New York, 2003. [2] K.T. Chung, S.E. Stevens, Degradation azo dyes by environmental microorganisms and helminths, Environ. Toxicol. Chem. 12 (1993) 2121–2132. [3] F.P. van der Zee, S. Villaverde, Combined anaerobic–aerobic treatment of azo dyes – a short review of bioreactor studies, Water Res. 39 (2005) 1425–1440. [4] M.A. Brown, S.C. De Vito, Predicting azo dye toxicity, Crit. Rev. Environ. Sci. Technol. 23 (1993) 249–324. [5] L.C. Apostol, L. Pereira, R. Pereira, M. Gavrilescu, M.M. Alves, Biological decolorization of xanthene dyes by anaerobic granular biomass, Biodegradation 23 (2012) 725–737. [6] S.S. Moghaddam, M.R. Alavi Moghaddama, M. Arami, Coagulation/flocculation process for dye removal using sludge from water treatment plant: optimization through response surface methodology, J. Hazard. Mater. 175 (2010) 651–657. [7] O. Türgay, G. Ersöz, S. Atalay, J. Forss, U. Welander, The treatment of azo dyes found in textile industry wastewater by anaerobic biological method and chemical oxidation, Sep. Purif. Technol. 79 (2011) 26–33. [8] D.E. Zavastin, S. Gherman, I. Cretescu, Removal of direct blue dye from aqueous solution using new polyurethane–cellulose acetate blend micro-filtration membrane, Rev. Chim. 63 (2012) 1075–1078. [9] J. Labanda, J. Sabaté, J. Llorens, Experimental and modeling study of the adsorption of single and binary dye solutions with an ion-exchange membrane adsorber, Chem. Eng. J. 166 (2011) 536–543. [10] P. Madhusudan, J. Zhang, B. Cheng, G. Liu, Photocatalytic degradation of organic dyes with hierarchical Bi2 O2 CO3 microstructures under visible-light, CrystEngComm 15 (2013) 231–240. [11] C.-R. Lee, H.-S. Kim, I.-H. Jang, J.-H. Im, N.-G. Park, Pseudo first-order adsorption kinetics of N719 dye on TiO2 surface, ACS Appl. Mater. Interfaces 3 (2011) 1953–1957. [12] T. Welton, Room-temperature ionic liquids. Solvents for synthesis and catalysis, Chem. Rev. 99 (1999) 2071–2084. [13] M. Fuerhacker, T.M. Haile, D. Kogelnig, A. Stojanovic, B. Keppler, Application of ionic liquids for the removal of heavy metals from wastewater and activated sludge, Water Sci. Technol. 65 (2012) 1765–1773. [14] M. Gharehbaghi, F. Shemirani, A novel method for dye removal: ionic liquidbased dispersive liquid–liquid extraction (IL-DLLE), Clean Soil Air Water 40 (2012) 290–297. [15] T. Poursaberi, M. Hassanisadi, Magnetic removal of reactive black 5 from wastewater using ionic liquid grafted-magnetic nanoparticles, Clean Soil Air Water, DOI: 10.1002/clen.201200160. [16] M. Min, L. Shen, G. Hong, M. Zhu, Y. Zhang, X. Wang, Y. Chen, B.S. Hsiao, Micronano structure poly(ether sulfones)/poly(ethyleneimine) nanofibrous affinity membranes for adsorption of anionic dyes and heavy metal ions in aqueous solution, Chem. Eng. J. 197 (2012) 88–100. [17] N. Li, R. Bai, C. Liu, Enhanced selective adsorption of mercury ions on chitosan beads grafted with polyacrylamide via surface-initiated atom transfer radical polymerization, Langmuir 21 (2005) 11780–11787. [18] R.B. Kasat, N.H.L. Wang, E.I. Franses, Experimental probing and modeling of key sorbent–solute interactions of norephedrine enantiomers with polysaccharide-based chiral stationary phases, J. Chromatogr. A 1190 (2008) 110–119. [19] G. Liu, M. Hou, J. Song, T. Jiang, H. Fan, Z. Zhang, B. Han, Immobilization of Pd nanoparticles with functional ionic liquid grafted onto cross-linked polymer for solvent-free Heck reaction, Green Chem. 12 (2010) 65–69.

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